The present invention relates to optical amplifiers, and more particularly relates to an ultra-wide bandwidth fiber based optical amplifier which divides the erbium wavelength band (1520 nm-1610 nm) into three separate bands, separately amplifies each of the three bands in parallel configuration, and then recombines the bands to provide uniform gain flatness over the entire bandwidth.
The design of wavelength division multiplexed (WDM) systems in the 1.5 .mu.m range is currently constrained by the limited bandwidth available from conventional erbium doped fiber amplifiers. The presently available bandwidth is limited to about 20 nm because of the highly structured gain spectrum of conventional erbium doped fibers. The use of gain equalization filters can extend the usable bandwidth up to about 40 nm (about 1525 nm to about 1565 nm). This 40 nm gain spectrum allows the use of more channels in a WDM system. However, proposed 10 Gb/s systems will require the use of the entire 80-90 nm bandwidth with very small channel spacings.
One possible solution to provide greater bandwidth would be to provide an erbium doped fiber that has a gain spectrum over a greater bandwidth. This would allow a single fiber amplifier to provide a gain spectrum over a greater bandwidth. Erbium doped fluoride fibers have shown gain spectrums of 25 nm without gain equalization filters, and newer, tellurite erbium doped fibers have gain spectrums in different ranges, but the gains are highly non-uniform. To date, it has been impossible to provide a single erbium doped fiber which has a uniform gain spectrum over more than a 25 nm bandwidth.
Another proposed solution is to divide the erbium bandwidth into two bands and separately amplify the separated bands in parallel configuration. This concept allows the use of two different amplifiers which can be optimized for a flat gain region within a specific band. This solution was proposed in the Apr. 10, 1997 publication of Electronics Letters (Vol. 33. No. 8). The article describes a broadband amplifier which divides the available bandwidth into two bands a 1520 nm-1570 nm band (1554 nm band) and a 1570 nm-1610 nm band (1.58 .mu.m band). The configuration of each band is based on a cascade configuration with a 980 nm pumped EDFA and a 1480 nm pumped EDFA using a combination of silica and fluoride fibers to optimize gain flatness. The EDFA unit for the 1.55 .mu.m band showed a relatively flat gain spectrum from 1530 nm-1560 nm, and the EDFA unit for the 1.58 .mu.m band showed a relatively flat gain spectrum from 1576 nm-1600 nm. The result is a wide bandwidth amplifier having a 54 nm flat gain spectrum. Although demonstrating an improved gain bandwidth of 14 nm over the prior single amplifier systems, this parallel configuration still loses significant bandwidth between the optimum gain spectrums, i.e. between 1560 nm and 1576 nm.
An 80 nm gain flattened amplifier using only silica erbium doped fibers was described in the Nov. 6, 1997 publication of Electronic Letters (Vol. 33 No. 23). Expansion of the gain flattened bandwidth from 54 nm to 80 nm was achieved by using two separate EDFA sections. The entire bandwidth is amplified in a first common section. After the first section, the optical channels are split into two bands, a C-band with a range of 1520 nm-1570 nm and an L-band with a range of 1570 nm-1620 nm. The C-band branch has a single stage amplifier, while the L-band branch has a two stage amplifier. The gain bandwidth in the C-band was shown to be 36.9 nm while the gain bandwidth in the L-band was shown to be 43.4 nm giving a total gain bandwidth of 80.3 nm. While the system demonstrates an even greater gain spectrum, the gain spectrum in both the C-band and L-band are non-uniform which makes real-life utilization of the entire gain spectrum difficult. The author's solution to improve gain spectrum flatness in the L-band is to change the inversion level, however, this comes at the expense of bandwidth. Accordingly, the entire 80 nm bandwidth would not be usable in an actual commercial device.
Furthermore a significant concern which prevents practical implementation of these proposed parallel designs is the problem of multipath interference (MPI) which is a phenomenon which naturally occurs when recombining two or more wavelength bands into a single fiber. Neither article discusses the problem or mentions an), possible solution to the problem.
Accordingly, while there have been attempts to provide a wide bandwidth amplifier having a greater gain spectrum, none of the present solutions solves the ultimate challenge of providing uniform gain flatness over the entire 1.5 .mu.m bandwidth.